1. Field of the Invention
This invention relates generally to upconversion laser systems, and more particularly, to a solid-state, continuous-wave, mode-locked laser system which produces output pulses at wavelengths significantly shorter than the pumping energy by energy pooling by trios of excited rare earth dopants.
2. Description of the Prior Art
A continuing challenge in ultrafast technology is the need for pulsed optical sources at short wavelengths for spectroscopy, communications, and metrology. There are currently no continuously mode-locked laser sources available at wavelengths shorter than that of the Argon ion laser at 458 nm. Continuous-wave (cw) mode-locked dye lasers operate only to .about.500 nm and solid state sources exist only at wavelengths longer than approximately 700 nm. Considerable research effort has therefore been directed to extending operation of short pulse infrared sources like the Ti:sapphire laser to shorter wavelengths through nonlinear harmonic generation. Also, widegap semiconductor lasers are under development. However, there have been no reports of mode-locking of short wavelength solid state lasers. Despite many desirable properties, solid state lasers typically operate at photon energies far below bandgap and suffer deleterious color center formation when short wavelength optical excitation is used. It is, therefore, no coincidence that even conventional optically-pumped, cw solid state lasers have not been demonstrated at short wavelengths.
Upconversion lasers, emitting at wavelengths shorter than their excitation, potentially avoid shortcomings of above-bandgap excitation of solid state media. It is, therefore, perhaps not too surprising that the first continuous-wave ultraviolet solid state laser was a Nd upconversion laser, illustrating the utility of long wavelength pumping. Recently, other upconversion lasers have been operated in the visible spectral region on the basis of cooperative upconversion and avalanche upconversion, and room temperature operation has been achieved in fibers and BaYYb.sub.0.99 Tm.sub.0.01 F.sub.8 crystals.
Currently there is interest in developing compact, short-wavelength laser sources for display and data storage applications. Widegap semiconductor diode lasers, harmonic generation by phase-matching or quasi-phase-matching, and upconversion lasers in bulk media and fibers are three promising approaches. However, widegap semiconductors present growth and doping problems which to date have prevented room temperature operation by current injection. Harmonic generation requires critical alignment in bulk crystals and suffers reduced efficiencies in fibers and slab waveguides. Upconversion lasers depend on complex internal dynamics to upgrade photon energy by mechanisms which are themselves still the subject of intense inquiry. Hence, many questions remain to be answered before the limitations of these various approaches can be fully assessed.
Initial upconversion laser research revealed low efficiencies and requirements for liquid helium cryogenics. However, many different mechanisms of upconversion exist and high efficiency, high temperature continuous operation is undoubtedly achievable in suitable media with appropriate techniques. To date, high efficiency and room temperature operation have only been demonstrated under separate circumstances in selected laser crystals and fibers, respectively.
It is, therefore, an object of this invention to provide a laser source which produces controllable optical pulses at wavelengths shorter than that used to pump the gain medium.
It is another object of this invention to provide a laser source which operates in a continuous-wave mode to produce controllable optical pulses at wavelengths shorter than that used to pump the gain medium.
It is also an object of this invention to provide an upconversion laser source which can operate efficiently near room temperature.
It is a further object of this invention to provide an upconversion laser source which produces controllable optical pulses by a cooperative process involving energy pooling of trios of excited rare earth dopant ions in a laser medium.
It is additionally an object of this invention to provide a solid-state laser system which directly emits mode-locked pulses at short wavelengths.
It is yet a further object of this invention to provide a solid-state laser system which employs a cooperative inversion mechanism and provides greater reliability operating in pulsed mode than can be expected from alternative upconversion mechanisms.
It is also another object of this invention to provide a solid-state upconversion laser system which operates in continuous-wave, mode-locked fashion, in the green spectral region.
It is yet an additional object of this invention to provide a mode-locked, short-wavelength, solid-state laser.
It is still another object of this invention to provide a mode-locked, short-wavelength, solid-state laser which does not exhibit spiking during continuous-wave operation.
It is a yet further object of this invention to provide a mode-locked, short-wavelength, Q-switched laser.